This invention relates in general to the field of metal halide arc discharge lamps and, in particular, to miniature low watt metal halide lamps of 35 watts or less achieving high efficacy and controlled color temperature performance.
In a typical prior art metal halide lamp, an envelope of vitreous silica material defines an arc chamber which contains a fill of mercury, inert gas, and metal halide. Sealed in the arc chamber is a pair of refractory tungsten electrodes having tips spaced apart from one another. After an arc discharge is established between the electrode tips, the temperature of the arc chamber rapidly increases, causing the mercury and metal halide to vaporize. The mercury atoms and metal atoms of the metal halide are ionized and excited, causing emissions of radiation at spectrums characteristic of the respective metals. This radiation is substantially combined within the arc chamber to produce a resultant light output having an established intensity and color temperature.
The color temperature and efficacy (usually expressed in terms of lumens per watt) are primarily dependent upon the vapor pressure of the halides in the arc chamber during lamp operation. Halide vapor pressure is strongly affected by the temperature of the wall of the envelope defining the arc chamber.
As is typical in prior art lamps, the metal halide does not entirely vaporize during operation. In fact, a noticeable condensate exists in the cooler regions of the arc chamber. It has been long understood that this halide condensation, particularly in lower wattage lamps, can significantly reduce efficacy and increase color temperature to unacceptable levels. Moreover, for double-ended lamps, halide condensation generally occurs at the opposing ends where the electrodes emerge from the vitreous silica material. These end regions are normally the coolest in the arc chamber. For double-ended lamps, this result is especially disadvantageous in that the temperature of these end regions are sensitive to manufacturing variations and variations occurring over time. Hence, the efficacy and color temperature performance of these lamps can vary significantly over their lifetime and from one lamp to another. Such variations are unacceptable in many applications.
Various attempts have been made to reduce the halide condensation in the end regions of the arc chamber. For example, Cap et al. U.S. Pat. No. 4,161,672 discloses that by reducing the cross-sectional area of the end shanks of the lamp envelope, the thermal loss through these shanks can be reduced. Cap et al. also discloses the use of opaque coatings of zirconiumoxide at the end regions to retain heat within the chamber. French et al. U.S. Pat. No. 4, 808,876 and Waymouth et al. U.S. Pat. No. 3,324,332 also disclose the use of end coatings and reduced dimensions in the envelope end seals or shanks. In addition, French et al. and Waymouth et al. disclose the use of end chambers or wells at the ends of the arc chamber. The wells have a reduced cross-section from the main body of the arc chamber to increase the temperature at the end regions.
In another example, Holle et al. U.S. Pat. No. 4,202,999 discloses that by reducing the physical size of the electrodes of miniature metal halide lamps, the heat loss through them is reduced, resulting in higher operational temperatures and higher efficacy.
In all of the above examples, the various techniques described have not been sufficient to adequately reduce halide condensation in the end regions of the arc chamber. In each example, the disclosed lamp design requires that the tips of the electrodes be relatively close to the end regions in order to maintain an adequate vaporizing temperature in these regions. Therefore, the distance over which the electrodes can be inserted into the arc chamber (i.e. insertion depth) is restricted in these prior art metal halide lamps. Such a restriction on insertion depth necessarily imposes a limit on the spacing between the electrode tips (assuming acceptable wall loading requirements must be maintained). As will be described below, this limitation can result in low efficacy levels for miniature metal halide lamps having input power ratings of 35 watts and below.